1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) switch for controlling traffic and, more particularly, it relates to an asynchronous transfer mode switch adapted to confine the transfer cell forwarding rate to a preselected forwarding rate.
2. Description of the Related Art
According to the ATM exchange technology, information from various different media including audio information and video information can be transmitted for telecommunications by using as units fixed length blocks that are referred to as cells. Such cells typically have a 53-octet length, of which a 5-octet length is used as a header section and the remaining 48-octet length is used as a user information section. Information to be used for identifying the destination of connection is included in the header section to realize a high speed switching by means of hardware. Additionally, since information can be transferred whenever it needs to be forwarded with the ATM, the existing rate can be utilized effectively and efficiently.
In a prior art ATM switch, an unspecified bit rate (UBR) type switched virtual connection (SVC) is used, so that a plurality of connections can be assigned to one queue of a shaping processing section, if the connections have the same declared rate. This will be explained later in detail.
Note that JP-A-8-125668 titled “ATM interface and Shaping Method” describes a technique, with which a “shaping” operation is performed by storing in advance the largest cell forwarding interval corresponding to each traffic condition that gives rise to an ATM cell portion discarded by the usage parameter control (UPC) for the virtual path identifier (VPI) and also for the virtual channel identifier (VCI) and, upon arrival of the ATM cell to be forwarded, forwarding it to an idle forwarding time slot according to the stored largest cell forwarding interval.
Also, JP-A-8-288951 titled “ATM Switch and Congestion Control Method for ATM-LAN” described a technique of selecting a backward explicit congestion notification (BECN) system for dealing with congestion by narrowing the transmission rate all the regions including the ATM local networks (LANs) containing respectively a transmission terminal and a reception terminal and also covering the transmission terminal and the reception terminal upon receiving a notification from a congested ATM switch depending on the site of congestion, a fast reservation protocol (FRP) system for dealing with congestion by forwarding a signal cell adapted to broaden the transmission rate for the downstream upon receiving a notification from the congested ATM switch and a forward explicit congestion notification (FECN) system for writing the identifier for “the occurrence of congestion” in the payload type (PT) of the cell header forwarded from the congested ATM switch and causing the reception terminal receiving the identifier to notify the transmission terminal so as to broaden the transmission rate, whichever is optimum, depending on the conditions of connection.
The prior art ATM switch, however, does not have a simplified configuration and is not adapted to assign optimum queues by using the UBR type SVC system. That is, if there are requests for connection requiring queues exceeding the number of queues constituting a cell buffer it has for shaping, it is impossible to effectively control the communication traffic.